Lithium (Li) ion electrochemical cells have high energy density and are commonly used in a variety of applications which include consumer electronics, wearable computing devices, military mobile equipment, satellite communication, spacecraft devices and electric vehicles. Lithium ion cells are particularly popular for use in large-scale energy applications such as low-emission electric vehicles, renewable power plants and stationary electric grids. Additionally, lithium ion cells are at the forefront of new generation wireless and portable communication applications. One or more lithium ion cells may be used to configure a battery that serves as the power source for these applications. The explosion in the number of higher energy demanding applications and the limitations of existing lithium ion technology are accelerating research for higher energy density, higher power density, higher-rate charge-discharge capability, and longer cycle life lithium ion cells. Today's commercialized lithium ion cells employ lithium intercalation materials for both the cathode and the anode.
Lithium ion cells are mainly composed of an anode, for example, graphite, a carbonate-based organic electrolyte, and a cathode comprising a cathode active material, for example, lithium cobalt oxide (LiCoO2). Lithium ions are intercalated and deintercalated between the anode and the cathode through the electrolyte during discharge and charge. When the cell supplies power, or is discharging, lithium ions generally move from the negative electrode (anode) to the positive electrode (cathode). When the cell is storing energy for later use, or is charging, the opposite occurs. Lithium ions generally move from the positive electrode (the cathode) to the negative electrode (the anode) during charging. For the example, the theoretical capacities of a graphite anode and a LiCoO2 cathode are 372 mAh/g and less than 160 mAh/g, respectively. These theoretical charge capacities, however, are too low for the recent surge in higher energy demanding applications.
Since it was first demonstrated that lithium metal can electrochemically alloy with other metals at room temperature, lithium alloying reactions with metallic or semi-metallic elements and various compounds have been investigated during the past few decades. Of the various lithium alloying elements studied for use in lithium ion cells, silicon (Si) has been considered one of the most attractive anode materials, because of its high gravimetric and volumetric capacity, and because of its abundance, cost effectiveness, and environmentally benign properties.
Prior art electrochemical cell electrodes are generally formed by mixing active electrode materials along with a solvent and binder material. The addition of the binder material, typically a polymeric binder, is added to hold the active electrode materials together. The binder acts like a glue that keeps the active electrode materials together and causes the materials to adhere to the current collector during electrochemical cycling. Furthermore, the binder enables the formation of the electrode shape during electrode manufacturing. As much as 15 weight percent binder may be used in a typical electrode fabrication process. The addition of this binder or binders provides an electrically inactive material that generally does not enhance, and in some cases may degrade the electrical performance of a resultant electrode within an electrochemical cell.
For example, polyvinylidene fluoride (PVDF) has conventionally been used as a prior art binder that is incorporated within a mixture of active electrode materials. This specific prior art binder was selected due to its resistance to volumetric swelling when exposed to electrolytic solutions typically found in a number of commercially available lithium ion batteries. However, this binder provides poor adhesion between active electrode materials and between the active electrode material and current collector. Thus, a large amount of binder is generally required for practical use. As a result, the capacity and energy density of the lithium ion secondary battery that utilize such binders typically decreases. In addition, because N-methylolpyrrolidone (“NMP”) is generally an expensive organic solvent, manufacturing an electrode and/or an electrochemical cell may become cost prohibitive. Furthermore, special safety and environmental precautions are generally needed to be taken when working with NMP, particularly when preparing the electrode slurry mixture, and when attaching the final electrode to a current collector.
Water-dispersible styrene-butadiene rubbers (SBR) combined with a thickening agent of carboxymethyl cellulose (CMC) have been proposed to solve the problems as noted above. Such a combination of styrene-butadiene rubbers (SBR) and carboxymethyl cellulose (CMC) has been used because: (1) the SBR type dispersing element is inexpensive, (2) it is water dispersible, (3) it has working environment conservational advantages, and (4) adhesion between active electrode materials and adhesion between the active electrode materials and current collectors is favorable.